RUBY-2058 test driver on Ruby 2.7 (#1615)

Author: p-mongo

.evergreen/config.yml

@@ -641,6 +641,10 @@ axes:
         display_name: ruby-2.6
         variables:
            RVM_RUBY: "ruby-2.6"
+      - id: "ruby-2.7"
+        display_name: ruby-2.7
+        variables:
+           RVM_RUBY: "ruby-2.7"
       - id: "ruby-head"
         display_name: ruby-head
         variables:
@@ -742,7 +746,7 @@ buildvariants:
   matrix_name: "mongo"
   matrix_spec:
     auth-and-ssl: ["auth-and-ssl", "noauth-and-nossl"]
-    ruby: "ruby-2.6"
+    ruby: "ruby-2.7"
     mongodb-version: "*"
     topology: "*"
     os: rhel70

.evergreen/functions.sh

@@ -116,7 +116,7 @@ setup_ruby() {
     if true; then
 
     # For testing toolchains:
-    toolchain_url=https://s3.amazonaws.com//mciuploads/mongo-ruby-toolchain/`host_arch`/ce62fbb005213564a3da1041854da54df6615b2a/mongo_ruby_driver_toolchain_`host_arch |tr - _`_patch_ce62fbb005213564a3da1041854da54df6615b2a_5cfacdc857e85a3ef6647ad9_19_06_07_20_49_15.tar.gz
+    toolchain_url=https://s3.amazonaws.com//mciuploads/mongo-ruby-toolchain/`host_arch`/e7cf68d7146c09d54dfbe241c04aad3e3eadbb10/mongo_ruby_driver_toolchain_`host_arch |tr - _`_e7cf68d7146c09d54dfbe241c04aad3e3eadbb10_19_12_27_00_47_13.tar.gz
     curl --retry 3 -fL $toolchain_url |tar zxf -
     export PATH=`pwd`/rubies/$RVM_RUBY/bin:$PATH